Tank sensor to be tested on pad with crew on board as agency aims for July lift-off
Putting faith in its ability to fix the fuel-sensor problem that prevented a launch on 13 July, NASA has rescheduled the Space Shuttle return to flight mission, STS 114/Discovery, for no earlier than 26 July. A fully fuelled external tank (ET) test will be conducted on the pad on 26 July, with the possibility that the test will be run during the countdown with the crew aboard, and the chance of a launch if the test passes all requirements.
The more likely scenario would be a launch on 27 July, if the test is successful. Officially, NASA has until 31 July to launch STS 114. It could extend the window to 4 August, but this would restrict lighting conditions for imaging the orbiter during its ascent. The next window will open on 9 September and last until 24 September. “We’ve waited two and a half years to be here and we’re trying hard to resolve this issue,” says NASA’s Shuttle programme manager Wayne Hale.
The eight fuel-level sensors in the ET are part of the engine cut-off (ECO) system that commands engine shutdown at the end of a nominal launch. There are four sensors each for the liquid oxygen and liquid hydrogen systems. All must be functioning for lift-off to be allowed. The No 2 liquid hydrogen sensor malfunctioned during the countdown for the STS 114 launch on 13 July, failing to switch from “wet” to “dry” during a test that involved sending signals to simulate a dry tank.
Inspections found nothing out of the ordinary and the problem could be just a loose wire, as the sensor continued to read “wet” for 3h before switching to “dry”. If the sensor fails again during the 26 July test, it may be necessary to swap wires leading from the ECO control box. If it malfunctions again, the sensor rather than the wiring may be at fault and this may require further work, causing more delays.
There were similar problems with ECO sensors during a tanking test on the pad in April, but the two sensors that caused the problem on that test were not involved in the 13 July countdown abort. After the April test, a point sensor box on Discovery was replaced with from one from Atlantis.
Another test followed and, after inspection, a sensor controller box was replaced with one from Endeavour.
TIM FURNISS/LONDON
Source: Flight International
